In the recovery of oil from subterranean formations, it is possible to recover only a portion of the oil in the formation using primary recovery methods utilizing the natural formation pressure to produce the oil. A portion of the oil that cannot be produced from a formation using primary recovery methods may be produced by Enhanced Oil Recovery (EOR) methods. There are three main types of EOR—thermal, chemical/polymer and gas injection—which may be used to increase oil recovery from a reservoir beyond what can be achieved by conventional means—possibly extending the life of a field and boosting the oil recovery factor.
Thermal enhanced recovery works by adding heat to the reservoir. The most widely practiced form is a steam drive, which reduces oil viscosity so that it can flow to the producing wells. Chemical flooding increases recovery by reducing the capillary forces that trap residual oil. Polymer flooding improves the sweep efficiency of injected water. Miscible gas injection works in a similar way to chemical flooding—by injecting a fluid that is miscible with the oil, trapped remaining oil can be recovered.
In miscible gas injection, a gas that is miscible with oil in place in the formation is injected into the formation. The injected gas dissolves in the oil forming a single phase solution with the oil, mobilizing the oil by swelling the oil, thereby reducing the viscosity of the oil, and by reducing the interfacial tension between the oil and water in the formation, thereby reducing the capillary forces that hold the oil in place in the formation. The mobilized oil may be displaced to a production well and recovered from the formation. Gases that have been utilized in miscible gas injection for enhanced oil recovery include ethane, propane, butane, liquid petroleum gases formed of C2 to C6 paraffinic hydrocarbons, synthesis gas (carbon monoxide and hydrogen), nitrogen, natural gas, and carbon dioxide.
In order for gas injection to provide effective recovery of oil from a formation by mobilizing the oil, the gas and the oil must be miscible under the formation conditions. The pressure and temperature of the formation, the nature of the oil in the formation, and the components of the gas determine the miscibility of the gas with oil in the formation. Gases that have components that are more similar to the oil in the formation are more miscible than gases that are less similar to the oil. Therefore, hydrocarbon-containing gases having higher carbon numbers (e.g. propane (C3) and butane (C4)) are more miscible in oil (typically formed of hydrocarbons having carbon numbers of C5 and greater) than methane (C1) or carbon dioxide. Higher formation pressure (e.g. >50 MPa) also facilitates miscibility of the gas with oil in the formation, where gases that have relatively low miscibility in the oil at low pressures may be miscible in the oil in the formation due to the pressure in the formation.
Associated gas is a form of natural gas frequently produced in conjunction with the production of oil from a formation. Associated gas is often considered a waste product, particularly when produced from remotely located formations having no access to a gas pipeline. Waste associated gas is commonly flared to dispose of the associated gas.
Associated gas is commonly comprised of at least 70 mol % methane, and may contain upwards of 75 mol %, or 80 mol %, or 90 mol % methane. Methane is miscible with oil at higher pressures (e.g 100 MPa at 100° C.), however, methane is immiscible with oil at lower pressures (e.g. less than 50 MPa) since the pressure is below the minimum miscibility pressure required to establish multiple contact miscibility of methane with the oil. As a result, associated gas or other gases formed primarily of methane have been unsuitable for use as a miscible gas for enhanced oil recovery from formations having a pressure of less than 50 MPa.
It is desirable to be able to utilize associated gas, or another hydrocarbon-containing gas, that contains a large mole fraction of methane as a gas in a miscible gas enhanced oil recovery process.